Generally, a mobile communication system provides a handover function such that a call can be continuously maintained though a mobile terminal departs from a service area (A) of a current base station and then advances into a service area (B) of an adjacent base station as shown in FIG. 1, thereby ensuring mobility.
Referring to FIG. 2, for conducting the handover function, when a pilot signal strength is detected to exceed a threshold value (t_add), for example, a mobile terminal (MS) 10 transmits PSMM (Pilot Strength Measurement Message) to a base station controller (BSC) 11, and the base station controller 11 analyzes PSMM and then instructs channel allocation to a corresponding base station if B sector (or, target sector) information exists in a neighbor list, and then transmits a handoff direction message to the mobile terminal 10 after the channel allocation is conducted. After that, the mobile terminal 10 adds a new pilot to an active set, and transmits a handoff complete message to the base station controller 11, thereby completing the handover processing.
In a synchronous CDMA network, pilots are managed in groups classified into an active set, a candidate set, a neighbor set and a remaining set. Here, the active set is a group of PN (Pseudo Noise) offsets associated with a traffic channel currently connected to the mobile terminal. The candidate set is a group of pilot offsets not belonging to a current active set but sufficiently capable of signal demodulation. The neighbor set is a group of pilot offsets not belonging to a current active set or a candidate set but having much probability of being belonged to a candidate set for handover. The neighbor set is defined and transmitted by a base station. The remaining set is a group of all pilots not belonging to the active set, the candidate set and the neighbor set.
For the mobile terminal to successfully conduct handover, information of a target sector corresponding to a source sector should exist in a neighbor list. Thus, it is very important to correctly make and update the neighbor list.
FIG. 3 shows a handover failure at points t and t at which handover is conducted when the information of a target sector does not exist in the neighbor list. As shown in FIG. 3, when a pilot signal strength of the B sector is detected to exceed a threshold value (t_add), the mobile terminal 10 transmits PSMM to the base station controller 11, and the base station controller 11 analyzes PSMM and then generates a call fault informing the handover failure if it is checked that a target sector does not exist in the neighbor list. This call fault may cause disconnection of a call, and it occupies about 20 to 30% of all call faults occurring in the system.
The neighbor list is generally made and updated based on manual operation of an operator, which thus takes much data analyzing time, gives possibility of subjective recording or errors, and also takes much time to verify a prepared neighbor list. Thus, there is a need for a scheme capable of automatically optimizing a neighbor list.
As a related technique, Korean Laid-open Patent Publication No. 2004-48038, entitled “Method for automatically updating a neighbor list in a CMDA communication network”, discloses increasing a handoff success rate by adding a handoff (handover) failed PN to a neighbor list using a neighbor PN statistical list, or by adjusting a neighbor list extract priority upward according to a handover trial frequency for the handoff-failed PN. However, this technique executes the neighbor list updating process per each base station controller (BSC), so it cannot receive statistical information on a PN not related to the corresponding BSC and thus it cannot update the information.
Meanwhile, Korean Laid-open Patent Publication No. 2005-24643, entitled “Method and apparatus for generating a handover neighbor list in a cellular mobile communication system” discloses determining priorities of subject base stations according to an order of handover occurrence frequencies and then generating a neighbor list according to the order. However, this technique has a limit in analyzing works since it uses handover statistical information per each base station, so it is difficult to give a highly reliable list. In addition, since it is difficult to get accurate location information of a base station not existing in a neighbor list but substantially included in statistics, it hardly calculates an accurate neighbor list, and also it cannot analyze a call fault generated when there is a handover request for a sector in the remaining set.